The present invention relates to electron beam irradiation apparatuses and more particularly, to an electron beam irradiation apparatus having a plurality of detectors for detection of backscattered electrons.
A scanning electron microscope (SEM) representing one type of electron beam irradiation apparatus detects secondary electrons (SE's) and backscattered electrons (BSE's: hereinafter referred to sometimes as reflected electrons), the SE's and BSE's being given of from a sample under irradiation of an electron beam (hereinafter referred to as a primary electron beam or primary electrons) on the sample. By converting signals of the SE's and BSE's into images and line profiles, the sample can be observed, inspected or measured. Especially, the electron microscope is widely applied to measurement of semiconductor devices and therefore, techniques for the semiconductor device measurement are required to be applicable to patterns of a variety of contours.
JP-A-9-36019 (Corresponding to U.S. Pat. No. 5,708,276) discloses an electron beam irradiation apparatus having 4 reflected electron detectors. The 4 reflected electron detectors are arranged axially symmetrically to the center of an electron beam optical axis to detect reflected electrons given off in the respective directions. Reflected electrons given off in a low angle direction (reflected electrons passing through a trajectory relatively close to the surface of a sample in contrast to reflected electrons passing through a trajectory paraxial to the electron beam optical axis) change their emission directions in accordance with directional situations of edges of a pattern, so that a shade contrast of the pattern can be formed into a distinct image through the use of the plural reflected electron detectors.
On the other hand, in order to detect reflected electrons with high efficiencies, a high voltage of about 10 kV needs to be applied to the detector. With the plural detectors arranged axially symmetrically to the electron beam optical axis as indicated in JP-A-936019, influences the electric fields based on applied voltages have upon the electron beam become axially symmetrical in theory. The inventors, however, studied to find out a possibility that actually, the influences become asymmetrical. This is considered to be caused by errors in assembling the electron microscope and by disturbance external of the electron microscope. The electron microscope will hereafter be required of further higher accuracy and therefore, there is a possibility that an axial misalignment or shift and aberration due to a slight asymmetric status of electric field cannot be neglected.